Shear cutter drill bit

ABSTRACT

A shear cutter drill bit for drilling a subterranean formation is provided having a bit body; a bit face on the bit body having at least a center zone, an efficiency zone, and an outer zone, the efficiency zone being located between the center zone and the outer zone; and a plurality of cutters having a backrake angle located on the bit face; wherein a majority of the cutters located in the efficiency zone of the bit face have a more aggressive backrake angle than a majority of the cutters located in the center zone of the bit face and a majority of the cutter located in the outer zone of the bit face.

FIELD OF THE INVENTION

The present application relates generally to shear cutter drill bits fordrilling subterranean formations. More specifically, a high rate ofpenetration (ROP) shear cutter drill bit is provided having cuttersarranged at low backrake.

BACKGROUND OF THE INVENTION

Shear cutter drill bits have a plurality of fixed cutters, usuallycomprising polycrystalline diamond compacts (“PDCs”), located on the bitface that are set at a fixed backrake angle. The drill bit is attachedto the end of a drill string and is rotated by either rotating the drillstring from surface or by using a downhole motor to form a borehole in asubterranean formation.

The rate of penetration (ROP), which generally defines how fast a givendrill bit drills, and the durability of a drill bit depends on a numberof factors, for example, cutter densities, number of cutters, size ofcutters, number of blades, arrangement of cutters on the bit face, andthe individual cutter's backrake angle. A drill bit that drills with ahigh ROP is generally referred to as an aggressive drill bit. Theoverall aggressiveness of a drill bit is generally defined by theaggressiveness of the cutters disposed on the face of the bit, i.e., howaggressively each cutter will bite into the formation. One of theprimary factors that determine the aggressiveness of an individualcutter is the backrake angle at which it is set. Backrake is defined asthe angle between the axis of the cutter and the formation that it iscutting, and, in general, the smaller the backrake angle, the moreaggressive the cutter.

Traditionally, the industry has used relatively high backrake angles, asbits with higher backrake tend to suffer less damage than bits withlower backrake. The industry standard has been to have cutters withbackrake angles above 20°. It is generally believed that drill bitshaving cutters with backrakes lower than 20° will be too aggressive,which will lead to premature cutter breakage. It is further believedthat drill bits with low backrake have poor durability as they presentless diamond surface area at the cutter—rock interface. However, use ofsuch high backrake cutters results in relatively unaggressive drill bitsrequires greater weight on bit (“WOB”) in order to achieve a higher ROP.

U.S. Pat. No. 7,000,715 describes a drill bit having cutters with higherbackrakes primarily located in the center of the bit, with the backrakesgetting generally lower with increasing radial position of the cutterson the bit face, with the lowest backrake cutters being positioned onthe shoulder of the bit. However, having cutters with the lowestbackrake present on the shoulder of the bit may lead to premature cutterbreakage. In addition to the higher likelihood of breakage on the lowbackrake shoulder cutters, breakage also becomes more of an issue in theother regions of the bit, in particular, the nose region, due toexcessive axial loading on these regions. U.S. Pat. No. 7,000,715 alsodescribes a ‘fast-drilling’ drill bit having cutters with the lowestbackrake located in the center of the bit, with the backrakes gettinggenerally higher with increasing radial position of the cutters on thebit face, with the highest backrake cutters being positioned on theshoulder of the bit. However, having cutters with the lowest backrakepresent in the center of the bit may also lead to premature cutterbreakage, and having higher backrake further out on the profile inhibitsthe bit's ability to drill at a high ROP. Further, in general, it hasbeen discovered that it is the cutters in the nose region that dictatethe efficiency of the overall cutting action of the bit rather than thecutters located outside of the nose region.

There is a need for a shear cutter drill bit which can drill with a veryhigh ROP without compromising the durability of the bit (i.e.,minimizing cutter breakage). The present applicants have discovered thatproviding a specific arrangement of cutters with low backrakes on thedrill bit face will result in fast (aggressive) drill bits that arestill durable.

SUMMARY OF THE INVENTION

In one broad aspect, the present application provides a shear cutterdrill bit for drilling a subterranean formation, having:

-   -   a bit body;    -   a bit face on the bit body having at least a center zone, an        efficiency zone, and an outer zone, the efficiency zone being        located between the center zone and the outer zone; and    -   a plurality of cutters having a backrake angle located on the        bit face;        wherein a majority of the cutters located in the efficiency zone        of the bit face have a more aggressive backrake angle than a        majority of the cutters located in the center zone of the bit        face and a majority of the cutter located in the outer zone of        the bit face.

In one embodiment, the majority of the cutters located in the efficiencyzone have a backrake angle less than 20°. In another embodiment, themajority of the cutters located in the efficiency zone have a backrakeangle less than 15°. In another embodiment, the majority of the cutterslocated in the efficiency zone have a backrake angle less than 10°. Inanother embodiment, the majority of the cutters located in theefficiency zone have a backrake angle less than 5°.

In another broad aspect, the present application provides a shear cutterdrill bit where a majority of the cutters located in the efficiency zonehave the most aggressive backrake angles, the majority of the cutterslocated in the center zone have intermediately aggressive backrakeangles, and the majority of the cutters located in the outer zone havethe least aggressive backrake angles. In one embodiment, the majority ofthe cutters located in the efficiency zone have a backrake angle in therange of about 5° to about 15°, the majority of the cutters located inthe center zone have a backrake angle in the range of about 15° to about20°, and the majority of the cutters located in the outer zone have abackrake angle in the range of about 15° to about 30°.

In another broad aspect, the application provides a method for drillinga subterranean formation, comprising:

-   -   providing a shear cutter drill bit for drilling a subterranean        formation, the shear cutter drill bit having a bit body, a bit        face on the bit body having at least a center zone, an        efficiency zone and an outer zone, the efficiency zone being        located between the center zone and the outer zone, and a        plurality of cutters having a backrake angle located on the bit        face, wherein a majority of the cutters located in the        efficiency zone of the bit face have a more aggressive backrake        angle than a majority of the cutters located in the center zone        of the bit face and a majority of the cutters located in the        outer zone of the bit face;    -   positioning the face of the drill bit towards the subterranean        formation so that at least one of the center zone, efficiency        zone and outer zone contacts the subterranean formation; and    -   rotating the drill bit while applying a weight on the drill bit        so as to penetrate the subterranean formation;        wherein the weight applied to the drill bit is less that the        weight needed to be applied to a conventional shear cutter drill        bit for obtaining the same rate of penetration.

Other features will become apparent from the following detaileddescription. It should be understood, however, that the detaileddescription and the specific embodiments, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an end view of an embodiment of a shear cutter drillbit according to the invention.

FIG. 2 illustrates the side perspective view of the drill bit embodimentshown in FIG. 1.

FIG. 3 illustrates a side view of a cutter as used in an embodiment ofthe invention depicting the cutter backrake.

FIG. 4 is a side cross-sectional elevation of an embodiment of a fivebladed shear cutter drill bit according to the invention.

FIG. 5 graphically illustrates a profile of the drill bit embodimentshown in FIG. 4 highlighting cutter height versus bit radius.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention is described below with reference to what arepresently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. To the contrary, the invention herein is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims.

The applicants have discovered that the cutters of a shear cutter drillbit that are located generally in a region adjacent to the cone regionof the bit and which extends partially into the shoulder region of thedrill bit appear to have the greatest impact on the overall efficiencyof the shearing action of the drill bit. This region is referred tohereinafter as the “efficiency zone”. It is understood by a personskilled in the art that the cone region of a drill bit generally refersto the center region of the drill bit (hereinafter referred to as the“center zone”) and that the shoulder region generally refers to theregion that starts where the profile angle is approximately 15°, theprofile angle being defined as the angle between a line drawnperpendicular to the bit profile and the centerline of the bit whenviewing the bit from a profile view.

Of course, it is understood to those skilled in the art that thebeginning of the efficiency zone will vary according to a number ofvariables: the intended RPM of the application, the type of rock beingdrilled, the overall size of the drill bit, the shape of the drill bitprofile, etc. However, for example purposes only, and not meant to belimiting, the efficiency zone generally starts at a radius ofapproximately 1″ from the centerline of the drill bit and, in general,the efficiency zone ends where the profile angle is approximately 45° inthe shoulder region.

It was further discovered that the beginning of the efficiency zonegenerally correlates to a radial distance where the relative surfacespeed of the cutters becomes more significant. The cutters located inthe center zone (i.e., closest to center of the bit) move at a very slowsurface speed (surface speed being defined as 2πRω, where “R” is theradial distance of the tip of the cutter to the center of the bit, and ωis the angular velocity—or RPM) with respect to the formation becausethey are positioned at such a small radial distance from the center ofthe bit. As a result of that slow surface speed, and as a result of thefact that these cutters are drilling rock that has been stress relieved,the cutters in the center generally have a reduced effect on theefficiency of the cutting structure. As the bit drills throughformation, the rock left in the center of the bit no longer has thestress of the rock around it to support it, and when rock is under lessstress, it is much easier to drill. In other words, the cutters do notneed to be very aggressive in the center zone for the overall drill bitto be aggressive.

Cutters located on the opposite side of the efficiency zone, i.e., thosecutters in the shoulder and gage region past the efficiency zone(hereinafter referred to as the “outer zone”), are oriented generallymore laterally than axially, and therefore have less effect on theefficiency of the axial cutting action (i.e., the ability of the bit todrill ahead). Because the cutters located in the outer zone are thefurthest away from the center of the cutter, these cutters have a highersurface speed with respect to the formation. Thus, cutters in the outerzone will still remove rock relatively fast even if they are not set ata particularly aggressive backrake angle, due to this increased surfacespeed.

Conversely, cutters in the efficiency zone are generally oriented moreaxially than laterally, and therefore have a greater effect on theefficiency of the axial cutting action. The rock being removed by thecutters near the nose of the bit is still supported by rock on threesides and, as such, is much harder to drill. Thus, it is desirable thatthe cutters in this zone be as aggressive as possible, i.e., have thelowest backrake, and that these lower backrake cutters extend past thebeginning of the shoulder of the bit. Hence, the cutters in theefficiency zone will have the greatest effect on the overall ROP of thedrill bit.

There may be additional advantages in having the cutters located in thecenter zone and outer zone set at a less aggressive angle. For example,the cutters in the center zone tend to be more susceptible to impactdamage and, in particular, can easily break if set at a low backrake.Thus, setting the cutters in the center zone at higher backrake angleswill tend to minimize such potential breakage without significantlyaffecting the overall drilling speed and efficiency of the drill bit.

With respect to the cutters in the outer zone, it is believed thathaving a high backrake on these cutters may help balance the axial loadson all of the cutters. Since the cutters in the efficiency zone willgenerally see the greatest amount of axial load (if all of the cutterson the drill bit were set at the same backrake), and since those cuttersare set at the most aggressive backrake, by increasing the backrake onthe cutters outside the efficiency zone it effectively increases theaxial load on those cutters outside the efficiency zone, thereforehelping to protect the cutters in the efficiency zone from excessiveaxial loading leading to cutter breakage.

Another possible advantage with high backrake angles on cutters in theouter zone may be that the cutters in that region tend to see the mostdamage from any lateral vibration or lateral movement of the bit. Sincelateral vibration is a common problem with shear cutter drill bits, theoverall performance of such drill bits may be improved by having thecutters in the outer zone at a higher backrake to prevent any breakageresulting from lateral movement/vibration.

Hence, since the efficiency of the overall cutting action of the bit isdictated by cutters in the efficiency zone, the cutters outside thatzone (either inside of it or outside of it) have less effect on theefficiency of the cutting action, and therefore less effect on theresulting ROP of the bit for a given WOB (weight on bit). Thus,according to one embodiment of the invention, the majority of thecutters located in the efficiency zone will have a more aggressivebackrake angle than the majority of the cutters located in the centerzone and the majority of the cutters located in the outer zone.

An embodiment of the invention will now be described with reference toFIGS. 1-5. With reference first to FIGS. 1 and 2, shear cutter drill bit20 comprises bit body 22 having a shank portion 24 and a bit face 26. Aplurality of generally radially oriented blades 28 generally define bitface 26 and each blade 28 further comprising a plurality of cuttingelements or cutters 30, each cutter having a particular backrake angleand disposed about the blades surface in a conventional fashion, e.g.,by brazing or force fitting. The most common type of cutters used in theindustry are cutters cylindrical in shape such as PDCs

The backrake angle of a cutter is defined as the angle between the axisof the cutter and the formation that it is cutting, measuredperpendicular to the profile of the drill bit. This can be seen moreclearly with reference to FIG. 3. In FIG. 3, the formation isrepresented by flat horizontal line 10, the axis of the cutter byhatched line 12, and the backrake angle of the cutter is the anglebetween the axis of the cutter and the formation (θ).

Drill bit 20 further comprises a plurality of nozzles or jets 32, whichprovide drilling fluid to the bit face 22. In FIG. 1, drill bit face 22is shown divided into three distinct regions by concentric hatchedcircles A and B, forming center zone 34, efficiency zone 36 and outerzone 38. Those cutters present in the efficiency zone 36 have beenmarked with an asterisk (*) in FIG. 2.

With reference now to FIG. 4, the profile of bit face 26 of drill bit 20as shown in FIGS. 1 and 2 is illustrated, including blades 28 upon whicha plurality of cutters 30 are oriented. The axis of the drill bit 20 isrepresented by hatched line 40 and the profile terminates at gage 42.Although all of the cutters 30 are depicted as being oriented on asingle blade 28, FIG. 4 merely depicts the position of cutters 30relative to one another with respect to both the longitudinal axis 40 ofthe bit 20 and a vertical position 44 along longitudinal axis 40. Inactuality, cutters 30 are carried on various blades 28, the cutterpositions having been rotated in a single plane for clarity. Zone 34,which extends from the axial line 40 to hatched line A, is the centerzone and it can be seen from the profile that, in this embodiment, thereare four cutters 30 in the center zone. Zone 36, which is the efficiencyzone, extends from hatched line A to hatched line B, and includes thenose point C. Zone 34, the outer zone, extends from hatched line B togage 42.

The profile of the blades 30 in FIG. 4 can be seen in more detail inFIG. 5. The x axis is taken along the longitudinal axis and generallyrepresents the distance of each cutter from the center of the bit or thebit radius. The y axis generally represents the height of the cutters.In this embodiment, the individual cutters have been labeled 30A to30CC, for a total of 29 cutters. Cutters 30A, 30B and 30C are present inthe center zone and in this embodiment have a backrake angle of about15°. Cutters 30D to 300 are present in the efficiency zone and in thisembodiment have a backrake angle of about 10°. Cutters 30P to 30CC arepresent in the outer zone. In this embodiment, cutters 30P to 30S have abackrake angle of about 15°, cutters 30T to 30W have a backrake angle of20°, and cutters 30X to 30AA have a backrake angle of about 30°.

It is understood, however, that the embodiment in FIG. 5 is only oneexample of a shear cutter drill bit of the invention. According to theinvention, generally, more than half of the cutters present inefficiency zone will have a backrake angle of about 20° or less, in someinstances about 15° or less, in some instances about 10° or less, or insome instances about 5° or less. The majority of the other cutters,which are located in the center zone and the outer zone, will havehigher backrake angles than the majority of those in the efficiencyzone.

For example, in another embodiment, the majority of the cutters locatedin the efficiency zone have a backrake angle in the range of about 5° toabout 15°; the majority of the cutters located in the center zone have abackrake angle in the range of about 15° to about 20°, and the majorityof the cutters located in the outer zone have a backrake angle in therange of about 15° to about 25°. In another embodiment, the majority ofthe cutters in the outer zone have a backrake angle of about 20° toabout 30°.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to those embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein, but is to beaccorded the full scope consistent with the claims, wherein reference toan element in the singular, such as by use of the article “a” or “an” isnot intended to mean “one and only one” unless specifically so stated,but rather “one or more”. All structural and functional equivalents tothe elements of the various embodiments described throughout thedisclosure that are known or later come to be known to those of ordinaryskill in the art are intended to be encompassed by the elements of theclaims. Moreover, nothing disclosed herein is intended to be dedicatedto the public regardless of whether such disclosure is explicitlyrecited in the claims. No claim element is to be construed under theprovisions of 35 USC 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or “step for”.

1. A shear cutter drill bit for drilling a subterranean formation,comprising: a bit body; a bit face on the bit body having at least acenter zone, an efficiency zone and an outer zone, the efficiency zonebeing located between the center zone and the outer zone; and aplurality of cutters having a backrake angle located on the bit face;wherein a majority of the cutters located in the efficiency zone of thebit face have a more aggressive backrake angle than a majority of thecutters located in the center zone of the bit face and a majority of thecutters located in the outer zone of the bit face.
 2. The drill bit asclaimed in claim 1, wherein the majority of the cutters located in theefficiency zone have a backrake angle less than 20°.
 3. The drill bit asclaimed in claim 1, wherein the majority of the cutters located in theefficiency zone have a backrake angle less than 15°.
 4. The drill bit asclaimed in claim 1, wherein the majority of the cutters located in theefficiency zone have a backrake angle less than 10°.
 5. The drill bit asclaimed in claim 1, wherein the majority of the cutters located in theefficiency zone have a backrake angle less than 5°.
 6. The drill bit asclaimed in claim 1, wherein the majority of the cutters located in theefficiency zone have the most aggressive backrake angles, the majorityof the cutters located in the center zone have intermediately aggressivebackrake angles, and the majority of the cutters located in the outerzone have the least aggressive backrake angles.
 7. The drill bit asclaimed in claim 1, wherein the majority of the cutters located in theefficiency zone have a backrake angle in the range of about 5° to about15°, the majority of the cutters located in the center zone have abackrake angle in the range of about 15° to about 20°, and the majorityof the cutters located in the outer zone have a backrake angle in therange of about 15° to about 30°.
 8. The drill bit as claimed in claim 1,wherein the majority of the cutters located in the efficiency zone havea backrake angle in the range of about 5° to about 15°, the majority ofthe cutters located in the center zone have a backrake angle in therange of about 15° to about 20°, and the majority of the cutters locatedin the outer zone have a backrake angle in the range of about 20° toabout 30°.
 9. The drill bit as claimed in claim 1, wherein the majorityof the cutters located in the efficiency zone have a backrake angle inthe range of about 10°, the majority of the cutters located in thecenter zone have a backrake angle of about 15°, and the majority of thecutters located in the outer zone have a backrake angle in the range ofabout 15° to about 30°.
 10. The drill bit as claimed in claim 1, the bitface comprising at least one blade, wherein the cutters are located onthe at least one blade.
 11. A method for drilling a subterraneanformation, comprising: providing a shear cutter drill bit for drilling asubterranean formation, the shear cutter drill bit comprising: a bitbody; a bit face on the bit body having at least a center zone, anefficiency zone and an outer zone, the efficiency zone being locatedbetween the center zone and the outer zone; and a plurality of cuttershaving a backrake angle located on the bit face; wherein a majority ofthe cutters located in the efficiency zone of the bit face have a moreaggressive backrake angle than a majority of the cutters located in thecenter zone of the bit face and a majority of the cutters located in theouter zone of the bit face; positioning the face of the drill bittowards the subterranean formation so that at least one of the centerzone, efficiency zone and outer zone contacts the subterraneanformation; rotating the drill bit while applying a weight on the drillbit so as to penetrate the subterranean formation; wherein the weightapplied to the drill bit is less that the weight needed to be applied toa conventional shear cutter drill bit for obtaining the same rate ofpenetration.
 12. The method as claimed in claim 11, wherein the majorityof the cutters located in the efficiency zone have a backrake angle lessthan 20°.
 13. The method as claimed in claim 11, wherein the majority ofthe cutters located in the efficiency zone have a backrake angle lessthan 15°.
 14. The method as claimed in claim 11, wherein the majority ofthe cutters located in the efficiency zone have a backrake angle lessthan 10°.
 15. The method as claimed in claim 11, wherein the majority ofthe cutters located in the efficiency zone have a backrake angle lessthan 5°.
 16. The method as claimed in claim 11, wherein the majority ofthe cutters located in the efficiency zone have the most aggressivebackrake angles, the majority of the cutters located in the center zonehave intermediately aggressive backrake angles, and the majority of thecutters located in the outer zone have the least aggressive backrakeangles.
 17. The method as claimed in claim 11, wherein the majority ofthe cutters located in the efficiency zone have a backrake angle in therange of about 5° to about 15°, the majority of the cutters located inthe center zone have a backrake angle in the range of about 15° to about20°, and the majority of the cutters located in the outer zone have abackrake angle in the range of about 15° to about 30°.
 18. The method asclaimed in claim 11, wherein the majority of the cutters located in theefficiency zone have a backrake angle in the range of about 5° to about15°, the majority of the cutters located in the center zone have abackrake angle in the range of about 15° to about 20°, and the majorityof the cutters located in the outer zone have a backrake angle in therange of about 20° to about 30°.
 19. The method as claimed in claim 11,wherein the majority of the cutters located in the efficiency zone havea backrake angle in the range of about 10°, the majority of the cutterslocated in the center zone have a backrake angle of about 15°, and themajority of the cutters located in the outer zone have a backrake anglein the range of about 15° to about 30°.
 20. The method as claimed inclaim 11, wherein the bit face comprising at least one blade and thecutters are located on the at least one blade.